GB2103822A - Flame retardant plastics sheathed optical and/or electrical cables - Google Patents

Description

1 GB 2 103 822 A 1

SPECIFICATION Plastics sheathed cables

This invention relates to plastics sheathed cables, particularly cables having an outer sheath which is designed to have flame retardant 70 properties.

Electric cables such as telephone cables are required in some circumstances to have flame retardant sheaths so that in the event of fire they do not readily burn. The cable is subject to a flammability test as laid down by the cable industry in this country, and this consists of holding the cable in a flame for 15 seconds to see if it burns or not. Flame retardancy can be achieved by flame retardant additives. A typical example of a flame retardant additive is sold under the trade name Hytrel HTR 4550.

However, when the flame retardant cable sheath is subject to the flammability test, although the cable sheath does not burn, it will nevertheless 85 melt and may expose the insulation of inner conductors which may not be flame retardant and which will thus burn.

According to the present invention there is provided a cable containing a conductor (optical 90 and/or electrical) within an outer plastics sheath of relatively low flammability material, the outer sheath containing at least 10-40% by weight of an inert particulate filler whose particle parameters are such as to minimise dripping of the sheath when subject to a flammability test as hereinafter defined.

In order that the invention can be clearly understood reference will now be made to the accompanying drawing which shows in Fig. 1 an optical fibre cable having a sheath in accordance with an embodiment of the present invention and in Fig. 2, schematically a flammability testing.

Referring to the drawing, Fig. 1, a cable comprises a central former 1 of high-strength plastics material or of metal around which are laid four mono-filament optical fibres such as 2. The fibres and central former are held together by a plastics wrapping tape 3 which, in this case, is 105 Mylar.

Around the Mylar tape is stranded some high strength plastics strands 4 which in this embodiment are made of a high-strength plastics 1 material sold under the trade name Keviar.

Around the Keviar strands is extruded a plastics sheath 5 which in this embodiment is a polyester elastomer solder under the trade name Hytrel, in particular Hytrel 5556. The sheath material also 115 contains a flame retardant additive, in this particular embodiment we have used Hytrel HTR 4450. The Hytrel HTR 4450 contains 10% bromine which imparts flame retardancy to the basic polymer by releasing halogens.

Fig. 2 shows schematically the cable of Fig. 1 undergoing a flammability test referred to herein as the test as herein defined. The cable is held at an angle of approximately 450 to the vertical in a propane flame 6 for approximately 15 seconds. This accords to a Ministry of Defence fibre optics cable flammability test. The temperature reached on the surface of the cable sheath may lie in the range of 300-6000C and it is a requirement that the cable sheath shall not continue to burn when it is removed from the flame. In order to meet this test the flame retardant additive, which as mentioned above is, in this particular embodiment, Hytrel HTR 4450, is incorporated in the sheath when it is extruded and basically acts as a flame retardant.

However, we have found that although the flame retardant properties of the cable are satisfactory the sheath nevertheless melts and drips and can drip to the extent that it exposes the Keviar strength members 4 underneath. This material burns readily and thus itself may sustain burning after the cable is removed from the flame.

Our tests have shown that if an inert filler having a suitable particle shape, range of sizes and particle size distribution, is added to the sheath, the tendency of the sheat to drip when subjected to the hot flame in the flammability test, can be significantly reduced, thus minimising the risk of exposure of more flammable material underneath the sheath.

In the particular embodiment described we used titanium dioxide powder for this purpose and the particular sheath composition was as follows:- Hytrel 5556 Hytrel HTR 4450 (flame retardant additive) titanium dioxide (anti-drip additive) parts parts parts with the titanium dioxide being mixed in with the sheath compound on a suitably heated twofoll mill.

We believe the non-drip filler acts when the sheath becomes molten to maintain the sheath in a gel-like state and imparts thixotropic properties to the sheath material.

In the embodiment described we believe that the addition of titanium dioxide in the range of 10-40% by weight of the sheath material would be effective. Below 10% will, in general, not impart sufficient thioxotropic properties to make a significant improvement whereas above 40% will begin to affect the physical properties of the finished sheath: for example, it will become too rigid for many applications.

In the embodiment described the sheath is extruded at a temperature of 2401C and the melt temperature is around 2000C. The filler used to prevent or minimise dripping must be inert both chemically with respect to the chemical composition of the sheath material and it must also have a significantly higher melting point than the basic sheath material.

The density of the non-drip additive should not be significantly greater than the sheath material.

The sample of titanium dioxide which we used 2 GB 2 103 822 A 2 is sold by British Titan Products Company Limited under their code No. T, OXIDE R-CR2. An 25 analysis of this material gave the following:- Mean particle size = 0.22p Standard Deviation = 0.10 t Distribution = 0.06 ft to 0.53 p we believe the range of particle sizes which would be satisfactory would be 0 to 10 i and a mean 35 particle size of the order of 0.5 ju.

The better the non-drip additive is distributed throughout the polymer the more effective it becomes.

In the embodiment described the optical fibre cable has an outer diameter of 5 mm, the sheath thickness being of the order of 1 mm and the optical fibre package at the Mylar tape interface having an outer diameter of about 2.5 mm.

Claims (8)

1. A cable containing a conductor (optical and/or electrical) within an outer plastics sheath ol relatively low flammability material, the outer 50 sheath containing at least 10-40% by weight of an inert particulate filler whose particle parameters are such as to minimise dripping of the sheath when subjected to a flammability test as hereinbefore defined.

2. A cable as claimed in claim 1, comprising a further material between the conductor and the sheath of a relatively high flammability. 30

3. A cable as claimed in claim 2, wherein the relatively high flammability material between the conductor and the sheath comprises stranded plastics material.

4. A cable as claimed in claim 3, wherein the stranded plastics material forms the main tensile strength member of the cable.

5. A cable as claimed in any preceding claim, comprising a plurality of optical fibres disposed around a central former and a tape wrapping around the fibres, the relatively high flammability material being laid on top of the wrapping and the sheath having been extruded over the high flammability material.

6. A cable as claimed in any preceding claim, wherein the inert filler is titanium dioxide.

7. A cable as claimed in any preceding claim, wherein the outer sheath has a composition as set out in the specific example referred to in the description.

8. A cable substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawing.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1983. Published by the Patent Office 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.